[quote=@Lord Wraith]
So my question then becomes, if Batman or Captain America were introduced today would Robin/Bucky still exist?
[/quote]

Kid Loki was introduced as a kind of Thor sidekick for Fear Itself and has his own fan following that rivals Lady Loki's. And I'd argue the Kid Loki storyline is one of the best told storylines that I've seen Marvel pull off in the last decade.

For a long time after Death in the Family, we didn't have a Robin. Tim was added and people are still upset that he's not Robin currently (isn't he still Robin in Batman & Robin Eternal though)? Damian came along and had this instant like him/hate him effect, which has followed the character throughout DCU, New 52, and whatever we're calling the state of DC comics today (50 fucking retcons later...) And they killed Damian and that had its own polarizing effect on the fan base, but at the end of the day people were still [b]demanding[/b] that there be a Robin. They're just split on whether Robin should be Tim or Damian (and then there's the Steph and Kelly Carrie fans)

So, to answer the question, I think the answer to the question is still yes. Young heroes have always stood on their own, from Jim Hawking in Treasure Island to Billy Batson in Captain Marvel to Louise Simonson creating an entire superhero team composed of 12 and under characters (Power Pack, which ran in its own series from 84-92). At the same time, the sidekick has become iconic and the young sidekick is just another trope related to that.